The present invention relates a multiplex transmission method and system and, more particularly, to a multiplex transmission method and system used in a high-speed optical subscriber system for distributing larger-volume signals, each obtained by multiplexing various signals and having a volume of G (Giga) bits/s.
Efforts have recently been made to standardize the specifications of B-ISDNs (Broadband Integrated Services Digital Networks) using the asynchronous transfer mode (to be referred to as the ATM hereinafter) according to an ITU-T recommendation. In the ATM, AALs (ATM Adaptation Layers) are used to form signals having various signal rates into ATM cells to allow any signals to be handled in a transmission line in the same manner, thereby realizing high-speed network processing. To form various signals into ATM cells, a CLAD (Cell Assembly and Disassembly) LSI is used to assemble the signals into cells on AALs defined for the respective signals.
As an ATM multiplexing method, a method called cell multiplexing is used. In this method, a plurality of cells are multiplexed on the time axis in units of cells to realize a method of transmitting signals to a high-speed transmission line. FIG. 4 explains a conventional multiplexing method based on cell multiplexing. In general, on a transmission line such as a trunk system or a LAN (Local Area Network), ATM cells are mapped according to VC-3 on OC-3 to be transmitted. As an ATM cell multiplexing scheme, a system for relaying/transmitting synchronous transfer mode (to be referred to as STM hereinafter) signals through an ATM network has been proposed, as disclosed in Japanese Patent Laid-Open No. 7-99493 (reference 1).
Referring to FIG. 4, an existing network I/F section 51 converts an STM reception signal into transmission data and a transmission frame sync signal indicating the start position of an STM frame. The STM reception signal has an STM frame configuration constituted by a plurality of time slots. All or some of these time slots are divided into a plurality of groups, and each group is provided as a basic line.
A cell assembly section 52 starts counting time slot numbers in response to a transmission frame sync signal from the existing network I/F section 51, and recognizes specific basic lines for which the respective time slots have been provided in accordance with the count values. Transmission data from the existing network I/F section 51 are sequentially accommodated in ATM cells having different virtual path identifiers (to be referred to as VPIs hereinafter) and virtual channel identifiers (to be referred to as VCIs hereinafter) for the respective basic lines.
In this manner, the cell assembly section 52 assembles ATM cells each having, in its information field, information indicating the presence/absence of the start time slot of an STM signal frame on a basic line in the information field and pointer information indicating the position of the start time slot, and outputs the cells as an ATM transmission signal to an ATM network through a user network I/F section 53.
The ATM cells received from the ATM network are separated in units of basic lines by using VPIs and VCIs, and STM transmission signals are regenerated from the respective ATM cells by using the pieces of pointer information in units of basic lines.
More specifically, in the user network I/F section 53, an ATM reception signal is converted into cells according to a predetermined format to obtain reception cells. A cell disassembly section 54 disassembles the reception cells from the user network I/F section 53 on the basis of the VPIs and the VCIs to receive the resultant data as time slot data in units of basic lines. The received time slot data are multiplexed to output reception data having an STM frame configuration. In addition, a reception frame sync signal is generated on the basis of information indicating the presence/absence of a start time slot.
In the existing network I/F section 51, the reception data and the reception frame sync signal from the cell disassembly section 54 are converted into an STM transmission signal to be output. Note that the arrangements of the cell assembly section 52 and the cell disassembly section 54 are disclosed in detail in reference 1.
As described above, in the conventional system shown in FIG. 4, when various signals are to be transmitted after they are disassembled into ATM cells and accommodated, a high-speed network is realized, and the band utilization efficiency is increased in trunk transmission owing to a statistic multiplex effect.
In such a conventional multiplex transmission system, however, original signals are simply multiplexed in the form of cells on the transmission system (side) for an ATM network, and only necessary signals are simply demultiplexed into cells on the receiver (side) for the ATM network. For this reason, when this system is used for transmission in a subscriber system such as a PDS (Passive Double Star) system using an optical fiber, original signals must be multiplexed in the form of cells to be transmitted in units of subscribers in accordance with the signal output positions of a demultiplexing circuit in the receiver.
When, therefore, broadcast signals such as CATV (CAble TeleVision) signals are to be distributed to the respective subscribers, identical signals concurrently flow on a transmission line in large quantities, resulting in a decrease in transmission line utilization efficiency.
In addition, with an increase in signal rate for transmission to subscribers, a circuit for multiplexing cells becomes complicated, resulting in difficulty in realizing a high-speed operation.